1. Field of the Invention
The present invention relates to a multilayer substrate and a manufacturing method thereof.
2. Description of the Related Art
An example of a multilayer substrate and a manufacturing method thereof is described in Japanese Unexamined Patent Application Publication No. 2008-147254. As described in Japanese Unexamined Patent Application Publication No. 2008-147254, a method of manufacturing a multilayer substrate by stacking a plurality of resin films made of thermoplastic resin is known. Each of the resin films has a conductive pattern provided on a principal surface of the multilayer substrate. Each of the resin films includes a hole. A hole penetrates through the resin film in the thickness direction at a position corresponding to the conductive pattern on the principal surface. The hole is filled with conductive paste, thereby providing a conductive portion penetrating through the resin film in the thickness direction. The conductive portion penetrating through the resin film in the thickness direction is tapered.
FIG. 32 illustrates the multilayer substrate described in Japanese Unexamined Patent Application Publication No. 2008-147254. The multilayer substrate 100 includes an insulated substrate 1 formed by stacking resin films 10a, 10b, 10c, 10d, 10e, and 10f. An electronic component 2 is disposed in the multilayer substrate 100 such that the electronic component 2 is wrapped in the insulated substrate 1. Each of the resin films includes a conductive pattern 3 on only one of principal surfaces. Each of the resin films includes a conductive portion 4 therein to penetrate through the resin film in the thickness direction. The conductive portion 4 is arranged within a region where the conductive pattern 3 extends. The conductive portion 4 is tapered in cross section. The narrow side is connected with the conductive pattern 3. Thus, in a single resin film, the wide side of the conductive portion 4 is exposed at another principal surface without the conductive pattern 3.
Referring to FIG. 32, the resin films 10a, 10b, 10c, 10d, and 10e are stacked in the multilayer substrate 100 such that the conductive patterns 3 are arranged at the upper sides. Only the resin film 10f is stacked such that the conductive pattern 3 is arranged at the lower side, such that the conductive pattern 3 is exposed at the lowermost surface of the multilayer substrate 100. As described above, the stacking direction of only the resin film 10f is inverted. The conductive pattern 3 is not present between the resin film 10e and the resin film 10f. 
Each resin film includes the conductive pattern on only one of the principal surfaces inevitably for the convenience of the manufacturing method. The conductive portion 4 is tapered because the hole is made by laser processing from the surface not including the conductive pattern 3 of each resin film.
A mount land electrode must be provided on the upper surface of the multilayer substrate for mounting a surface mount device (also referred to as “SMD”) and an integrated circuit (also referred to as “IC”) on the upper surface, and a land electrode must be provided on the lower surface of the multilayer substrate for mounting the multilayer substrate itself on a motherboard or the other components. In general, when a multilayer substrate is formed by stacking resin films, the stacking direction of either of the uppermost and lowermost resin films must be inverted to cause conductive patterns to be exposed at both of the uppermost and lowermost surfaces.
In the multilayer substrate described above, the conductive portion 4 of the resin film 10e is directly connected with the conductive portion 4 of the resin film 10f such that the conductive pattern 3 is not interposed therebetween as shown in FIG. 32. However, a connection failure may occur at the connection position between the conductive portions 4.